Question

Cloud droplets form and grow as water vapor condenses onto a hygroscopic condensation nuclei. Research has shown that the maximum radius for cloud droplets is about 0.05 millimeter. However, typical raindrops have volumes thousands of times greater. How do cloud droplets become raindrops?

Short answer

Cloud droplets become raindrops primarily through collision-coalescence and, in cold clouds, the Bergeron process.

Step-by-step solution

Understand Cloud Droplets Formation

Cloud droplets are formed when water vapor condenses onto hygroscopic condensation nuclei, such as dust or salt particles. This initial condensation results in very small droplets with a maximum radius of about 0.05 millimeters.

Recognize the Limitation of Cloud Droplets

Due to their small size and weight, cloud droplets cannot reach the ground as rain. If they remain as small droplets, gravity is not strong enough to overcome air resistance, causing them to evaporate before reaching the ground.

Explain Collision-Coalescence Process

Raindrops form mainly through the collision-coalescence process. In this process, larger cloud droplets colliding with smaller ones eventually grow in size. As these droplets collide and coalesce, they form larger droplets capable of falling to the ground as raindrops.

Discuss Role of Updrafts

Air currents in clouds, known as updrafts, cause cloud droplets to be lifted higher. While suspended, they encounter even more droplets, enhancing the likelihood of further collisions and coalescence.

Explain Bergeron Process

In cold clouds, the Bergeron process can be a major factor. This involves the growth of ice crystals at the expense of supercooled water droplets. Over time, as ice crystals grow, they can fall as snowflakes or melt into raindrops during their descent to warmer air below.

Key concepts

Cloud Droplets

Cloud droplets are the tiny building blocks of raindrops. These droplets are formed when water vapor in the air condenses onto tiny particles known as hygroscopic condensation nuclei.
These nuclei can be anything from dust to salt particles.
Once the water vapor sticks to these particles, it forms small droplets with a radius of about 0.05 millimeters.

• These droplets are incredibly lightweight.
• They are often too small to collide with each other and grow further on their own.

Cloud droplets hover in the atmosphere, usually with the help of air currents to keep them aloft.

Condensation Nuclei

Condensation nuclei are the tiny particles that water vapor attaches to in the atmosphere to form cloud droplets.
These particles are crucial because they provide a surface for water vapor to condense upon, starting the process of droplet formation.
Without condensation nuclei, it would be much more difficult for cloud droplets to form.

• Common nuclei include volcanic ash, sea salt, and industrial pollutants.
• Natural events like volcanic eruptions can significantly increase their abundance in the atmosphere.

Their presence and properties greatly influence the formation and growth of cloud droplets.

Collision-Coalescence Process

The collision-coalescence process is a key mechanism for transforming cloud droplets into raindrops.
This process occurs when larger cloud droplets encounter and merge with smaller ones in the cloud.
Over time, these repeated collisions and mergers cause the droplets to grow significantly in size.

• Gravity aids this process by pulling larger droplets downward, where they're more likely to collide with others.
• Strong updrafts within clouds keep droplets suspended long enough for multiple collisions to occur.

Eventually, the droplets become large enough to fall to the ground as raindrops.

Bergeron Process

The Bergeron process primarily takes place in cooler clouds.
It involves both ice crystals and supercooled water droplets.
In these clouds, water vapor preferentially deposits onto ice crystals rather than liquid droplets, allowing the crystals to grow larger.

• Supercooled droplets lose mass to growing ice crystals.
• Once large enough, ice crystals can fall as snowflakes or melt into raindrops in warmer air below.

This process is especially significant in the formation of precipitation in colder climates, complementing the collision-coalescence mechanism in warm clouds.